'\" et
.TH SOCKATMARK "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
sockatmark
\(em determine whether a socket is at the out-of-band mark
.SH SYNOPSIS
.LP
.nf
#include <sys/socket.h>
.P
int sockatmark(int \fIs\fR);
.fi
.SH DESCRIPTION
The
\fIsockatmark\fR()
function shall determine whether the socket specified by the descriptor
.IR s
is at the out-of-band data mark (see
.IR "Section 2.10.12" ", " "Socket Out-of-Band Data State").
If the protocol for the socket supports out-of-band data by marking the
stream with an out-of-band data mark, the
\fIsockatmark\fR()
function shall return 1 when all data preceding the mark has been read
and the out-of-band data mark is the first element in the receive
queue. The
\fIsockatmark\fR()
function shall not remove the mark from the stream.
.SH "RETURN VALUE"
Upon successful completion, the
\fIsockatmark\fR()
function shall return a value indicating whether the socket is at an
out-of-band data mark. If the protocol has marked the data stream and
all data preceding the mark has been read, the return value shall be 1;
if there is no mark, or if data precedes the mark in the receive queue,
the
\fIsockatmark\fR()
function shall return 0. Otherwise, it shall return a value of \-1
and set
.IR errno
to indicate the error.
.SH ERRORS
The
\fIsockatmark\fR()
function shall fail if:
.TP
.BR EBADF
The
.IR s
argument is not a valid file descriptor.
.TP
.BR ENOTTY
The file associated with the
.IR s
argument is not a socket.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
The use of this function between receive operations allows an
application to determine which received data precedes the out-of-band
data and which follows the out-of-band data.
.P
There is an inherent race condition in the use of this function. On an
empty receive queue, the current read of the location might well be at
the ``mark'', but the system has no way of knowing that the next data
segment that will arrive from the network will carry the mark, and
\fIsockatmark\fR()
will return false, and the next read operation will silently consume
the mark.
.P
Hence, this function can only be used reliably when the application
already knows that the out-of-band data has been seen by the system or
that it is known that there is data waiting to be read at the socket
(via SIGURG or
\fIselect\fR()).
See
.IR "Section 2.10.11" ", " "Socket Receive Queue",
.IR "Section 2.10.12" ", " "Socket Out-of-Band Data State",
.IR "Section 2.10.14" ", " "Signals",
and
\fIpselect\fR()
for details.
.SH RATIONALE
The
\fIsockatmark\fR()
function replaces the historical SIOCATMARK command to
\fIioctl\fR()
which implemented the same functionality on many implementations. Using
a wrapper function follows the adopted conventions to avoid specifying
commands to the
\fIioctl\fR()
function, other than those now included to support XSI STREAMS. The
\fIsockatmark\fR()
function could be implemented as follows:
.sp
.RS 4
.nf

#include <sys/ioctl.h>
.P
int sockatmark(int s)
{
    int val;
    if (ioctl(s,SIOCATMARK,&val)==-1)
        return(-1);
    return(val);
}
.fi
.P
.RE
.P
The use of
.BR [ENOTTY] 
to indicate an incorrect descriptor type matches the historical
behavior of SIOCATMARK.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "Section 2.10.12" ", " "Socket Out-of-Band Data State",
.IR "\fIpselect\fR\^(\|)",
.IR "\fIrecv\fR\^(\|)",
.IR "\fIrecvmsg\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<sys_socket.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
